Composite ceramic material and method to manufacture the material

ABSTRACT

The present invention relates to a method to manufacture a composite ceramic material having a high strength combined with bioactive properties, when the material is used as a dental or orthopedic implant, which includes preparing a powder mixture, mainly comprising partly a first powder, which in its used chemical state will constitute a bioinert matrix in the finished material , and partly a second powder, mainly comprising a calcium phosphate-based material. The invention is characterized in that said first powder comprises at least one of the oxides belonging to the group consisting of titanium dioxide (TiO 2 ), zirconium oxide (ZrO 2 ) and aluminum oxide (Al 2  O 3 ), in that said second powder mainly comprises at least one of the compounds hydroxylapatite and tricalcium phosphate, in that a raw compact is made of said powder mixture and in that said raw compact is densified through an isostatic pressing in a hot condition (HIP) at a pressure higher than 50 MPa, a composite material being obtained, in which said matrix comprises one or several metal oxides of said first powder, in which matrix said compound hydroxylapatite and/or tricalcium phosphate is evenly dispersed. 
     The invention also relates to a composite ceramic material as well as a body, completely or partially made of this material.

TECHNICAL FIELD

The present invention relates to a composite ceramic material having ahigh strength combined with bioactive properties when the material isused as a dental or orthopedic implant. The invention relates also to amethod to manufacture the composite ceramic material.

BACKGROUND ART

Ceramic materials and particularly structural ceramic materialsgenerally have a high resistance to corrosion and erosion. This is trueof e.g. several oxides, nitrides, carbides and borides. Also, saidmaterials have no toxic properties. When used as implant materials saidmaterials are completely inactive, i.e. neither positive nor negativereactions with surrounding tissues take place, and consequently it ispossible to attain a biological integration to bone tissue without anyintermediate connective tissue. Such materials are termed inert whenused as implant materials. These properties make several oxides,nitrides, carbides and borides potentially very valuable as inert dentaland orthopedic implant materials.

However, it is desirable that materials having a favorablebiocompatibility are not only inert, i.e. able to fasten mechanically toa bone tissue, but also bioactive, i.e. the implant can be bondedchemically to a bone tissue. Oxides, nitrides, carbides and borides donot have this property. On the other hand it is known thatphosphate-based materials, having a chemical composition similar to the"inorganic" or "ceramic" matter in bone tissue, can display bioactiveproperties. Such a phosphate-based material is e.g. hydroxylapatite,Ca(PO₄)₃. However, a synthetic hydroxylapatite has a low tensiletoughness and hence a low strength and also a tendency to graduallydevelop a continuous crack growth. Another example of a bioactivematerial having a calcium phosphate-base is tricalcium phosphate Ca₃(PO₄)₂, but this compound has an unsatisfactory strength. Also, it has anot negligible water solubility and consequently may be dissolved beforethe bond to the bone tissue has developed. Thus, in this respect,hydroxylapatite is preferred as compared to tricalcium phosphate.

DISCLOSURE OF THE INVENTION

The object of the invention is to suggest a composite ceramic materialhaving a so called duo-quality , i.e. a high strength combined with abioactivity. This and other objects can be attained by using a material,which comprises, when it is used as an implant material, an inert matrixhaving a high strength and in the matrix evenly distributed 5 -35percent by volume of at least one second phase, which mainly comprisesat least one material having a calcium phosphate-base. This calciumphosphate-base can e.g. be hydroxylapatite and/or tricalcium phosphate,while the matrix can comprise mainly one or several oxides and/ornitrides. The matrix comprises preferably mainly one or several oxidesbelonging to the group which comprises titanium dioxide, zirconium oxideand aluminum oxide.

The used hydroxylapatite can be entirely synthetic or consist of a boneash, which also contains other compounds than hydroxylapatite in smallcontents.

The material is produced by preparing a powder mixture, which mainlyconsists of partly a first powder, which in the condition the powdermaterial is in during the admixture or it will obtain after a subsequentchemical reaction can form a biologically inert matrix in the finishedmaterial when it is to be used as an implant material, and partly asecond powder, mainly consisting of a material having a calciumphosphate-base. A raw compact is made of this powder mixture anddensified by a hot isostatic pressing at a temperature of 900°-1300° C.and a pressure higher than 50 MPa, preferably at a pressure of at least150 MPa and preferably not more than 250 MPa. The raw compact (greenbody) is in this case suitably produced by a cold isostatic compacting,which precedes the hot isostatic compacting, which is facilitated by theadmixture of the calcium phosphate powder into the oxide powder. Despitethe comparatively low temperature during the hot isostatic compacting itis possible to attain a density of at least 97%, if the matrix consistsof oxides. The comparatively low sintering temperature is advantageous,e.g. for the following reasons:

The grain growth is limited, which favors a high strength;

A decomposition of the calcium phosphate material (hydroxylapatite orthe like) is avoided or limited to an acceptable extent; and

Undesirable reactions between oxides, e.g. titanium dioxide, and thecalcium phosphates are prevented.

When the powder material is consolidated, no chemical reactions takeplace. The powder mixture suitably is composed in such a manner, thatthe calcium phosphate-phase will appear as small islands, i.e. asdiscrete particles, in the matrix, which according to the firstpreferred embodiment of the invention consists of one or several oxidesbelonging to the group, which comprises titanium dioxide, zirconiumoxide and aluminum oxide. One might expect that 5 -35 percent by volumeof calcium phosphate material in the inert matrix is not sufficient toensure the desired bioactivity but at the same time will result in arisk of a substantial deterioration of the strength properties. However,we have found that these fears are groundless. Thus, clinicalexperiments, performed on living animals, have shown, that the materialaccording to the invention has a bioactivity, which is entirelycomparable to the bioactivity of pure hydroxylapatite. Thus, it is notnecessary to coat the inert matrix with pure hydroxylapatite in order toobtain the desirable bioactivity, which otherwise is customary accordingto known practice. However, the admixture of calcium phosphate materialinto the matrix probably must be very finely dispersed and even in orderto obtain a very large number of islands per exposed surface unit, thedistance between, adjacent islands of calcium phosphate at the same timebeing very small. These conditions probably will facilitate the additionof a new bone tissue, but the causal relations have not been completelyexplained. The very finely dispersed and even nature of the distributionof the calcium phosphate fraction in the matrix can also explain theretained very high strength. Thus, provided one regards the calciumphosphate particles as defects of the matrix, it is possible tocalculate the largest possible size of the calcium phosphate islands indifferent oxide materials in order to obtain a certain so calledcritical intensity factor (critical toughness), known for the matrix.Thus, if the matrix is tougher, the islands can be larger than in abrittle matrix and vice versa. These theoretical considerations andpractical results lead to the following recommendations for a compositeconsolidated material consisting of a matrix, which comprises metaloxides, and a calcium phosphate material dispersed in the matrix,preferably hydroxylapatite.

    ______________________________________                                               Maximum size of                                                                            Mean distance between                                            the calcium phos-                                                                          the calcium phosphate                                     Matrix phate particles                                                                            particles                                                 ______________________________________                                        TiO.sub.2                                                                            10 μm     Max. 5 μm, pref.                                                                         max. 2 μm                                Al.sub.2 O.sub.3                                                                     15 μm     "             "                                           ZrO.sub.2                                                                            30 μm     "             "                                           ______________________________________                                    

Thus, as regards the strength, zirconium oxide is preferred to aluminumoxide, which in its turn is better than titanium oxide. Also, the oxidesdiffer as to their chemical resistance. Thus, whereas the temperatureduring the hot isostatic pressing should not be higher than 1000 °C.,when the matrix is composed of titanium dioxide, it can be as high as1300 °C. and preferably 1100-1250° C., when a matrix of aluminum oxideand/or zirconium oxide is used.

Whereas according to known practice one believes that it is necessary toprovide an implant with a coat, which entirely consists of a bioactivematerial, which means that in certain cases the implant has beenprovided with an outer coat of a calcium phosphate material, accordingto the present invention it is possible to use the duo-properties of thecomposite material according to the invention, alone and an extra outercoat of hydroxylapatite or the like is not being necessary. However, itis also, within the scope of the present invention, possible per se tocover a "duo-material", completely or partially, with layers ofmaterials having another composition or to produce implants, in whichdifferent parts have different compositions, including at least one partconsisting of a composite material according to the invention havingduo-properties and at least one part consisting of a homogenousmaterial, e.g. a ceramic material without any admixture of a calciumphosphate material or an entirely metallic material. These and otheraspects of the invention are also set forth in the patent claims andwill be further elucidated in the following description of a fewpreferred embodiments.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be explained in more detail by means of a fewillustrative examples and experiments carried out, reference being madeto the accompanying drawings, in which:

FIG. 1 shows a microstructure of a material according to a firstpreferred embodiment according to the invention;

FIG. 2 is a drawn picture, based on an X-ray photograph, which shows howan implant according to the invention has adhered to a natural bonematerial;

FIG. 3 shows a longitudinal section of a product according to a possibleembodiment of the invention;

FIG. 4 shows a longitudinal section of a product according to anotherpossible embodiment of the invention; and

FIG. 5 shows a longitudinal section of a product according to anadditional possible embodiment of the invention.

DESCRIPTION OF EXPERIMENTS CARRIED OUT AND EMBODIMENTS

Raw materials used in the trials are listed in Table 1. Monolithicmaterials (titanium dioxide and hydroxylapatite respectively) as well ascomposite materials (combinations of oxides and calcium phosphatematerials), see Table 2, were produced from powders of these rawmaterials. The powder mixtures and a silicon nitride grinding agent inpetroleum ether were admixed in a ball mill and were ground for 20 h.Subsequent to an evaporation in an evaporator raw compacts were producedfrom the powder mixtures by a cold isostatic compacting (CIP) at apressure of 300 MPa. The raw compacts thus obtained were encased inglass and densified by a hot isostatic pressing (HIP) at a pressure of200 MPa for 1 h at a maximum temperature of 925° C. for the TiO₂ - basedmaterials and at 1225° C. for the other materials. Subsequent to thedensification the materials displayed a density of more than 99% of thetheoretical maximum density. The HIP-temperatures and the obtaineddensities are also shown in Table 2.

                  TABLE 1                                                         ______________________________________                                        Raw materials                                                                 Designation                                                                             Description                                                         ______________________________________                                        BA        Hydroxylapatite of bone ash                                         HA        Hydroxylapatite, grade Merck                                        TCP       β-tricalcium phosphate, grade Merch                            A         α-aluminum oxide, AKP-30, Sumitomo                            R         Titanium dioxide, Tioxide Ltd                                       Z         Zirconium dioxide (including 3 mol % Y.sub.2 O.sub.3)               ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Powder mictures which have been compacted by a                                hot isostatic pressing; densities of HIP-produced specimens                                        HIP-                                                     Specimen                                                                             Powder,       temp.    Density g/cm.sup.3                              No.    % by volume   °C.                                                                             Recorded                                                                             Theoretical                              ______________________________________                                        1      70HA/30A      1225     3.39   3.40                                     2      25HZ/75A      1225     3.75   3.77                                     3      15BA/85A      1225     3.85   3.85                                     4      15HA/85R       925     4.02   4.09                                     5      7.5TCP/7.5HA/85A                                                                            1225     3.77   3.85                                     6      7.5TCP/7.5HA/85R                                                                             925     4.04   4.09                                     7      15HA/85Z      1225     5.64   5.65                                     ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                Tensile   Weibull-           Hardness                                 Specimen                                                                              strength  module    Toughness                                                                              (5N)                                     no.     (MPa)     (m)       (MPam.sup.1/2)                                                                         (GPa)                                    ______________________________________                                        HA      110       18        1.1 ± 0.1                                                                           3.9 ± 0.3                             1       250       n.d.*     2.0       7.1                                     2       535       n.d.*     4.0      20.2                                     3       601       19        3.5      18.9                                     4       252        9        2.9       8.4                                     5       446       10        3.4      17.8                                     6       397       n.d.*     2.6      10.9                                     7       820       n.d.*     >7       13                                       A       400-560   --        4        22                                       R       405       10        --       12                                       Z       980       n.d.*     >7       14                                       ______________________________________                                         *not measured                                                            

Test bars were made with the size 3×3×30 mm. The test bars were examinedin a three-point test to measure the compressive strength in bending.The Weibull-modules were measured. The toughness was measured usingVicker's indentation depth method, as well as the hardness at a load of10N och 5N respectively. Some specimens were etched for 20 seconds in a0.1% HF-solution in order to study the microstructure in SEM.

In order to study the bioactive properties of the materials cylinderswere made with a diameter of 3.1 mm and a length of 7 mm. Identicalspecimens of pure aluminum oxide (negative control) and purehydroxylapatite (positive control) were also made to be used asreference materials. The implants were inserted by operation in a largehole (3.2 mm diameter) in lateral cortex in rabbits (femora-rabbit franNew Zeeland). After a healing period of three months the animals wereput to death and the implants were examined by X-ray radiography,subsequent to the removal of surrounding soft tissue.

FIG. 1 shows the microstructure of a material according to theinvention, being composed of 15 percent by volume hydroxylapatite, theremainder being aluminum oxide (specimen 3). The hydroxylapatite-phaseis evenly distributed throughout the aluminum oxide-matrix, in which thehydroxylapatite forms particles (grains) or islands (insulets) having amaximum length of <6 μm . The specimen is somewhat overetched in FIG. 1in order to be able to identify the microstructure more easily. Some ofthe smallest grains can be pores. An X-ray diffraction analysis showedthat no phase alterations had taken place during the HIP-treatment.

The mechanical properties are shown in Table 3. As is expected, thealuminum oxide-based duo-ceramic materials, specimens 1, 2, 3 and 5, arestronger than the titanium dioxide based materials, specimens 4 and 6.The strength level of the aluminum oxide-based duo-ceramic materials iscomparable to the strength level of commercial dental implants made ofpolycrystalline aluminum oxide, which is 400-560 MPa. The tensilestrength is also comparable. The zirconium oxide-based duo-ceramicmaterials have the highest strength and tensile strength. The resultsshow, that the duo-ceramic materials according to the present inventioncan be used for dental implants, at least as regards the mechanicalproperties. This is particularly true of those duo-ceramic materialsaccording to the invention, which are based on aluminum oxide andzirconium oxide, but also the titanium-based duo-ceramic materials inall likelihood can be used as implants, at least in those instances whenthe mechanical properties are a critical factor.

FIG. 2 shows in a drawing, based on an X-ray radiograph a ceramiccylinder, made of specimen no. 3 in Table 3 and inserted by operation. Anew cortical bone material has grown towards the implant (at a) as wellas along the surface of the implant (at b). The pattern of bone growthfor the duo-ceramic material according to the invention is mainlyidentical with that for pure hydroxylapatite, as was shown in acomparison test. A similar pattern was also obtained with specimens no.4and no. 7, which had a matrix of titanium dioxide and zirconium oxiderespectively.

These trials show, that bioactive ceramic materials having a highstrength can be produced by means of a hot isostatic pressing of a rawcompact, which is composed of at least two powder fractions, a bioactivephase being obtained, which consists of hydroxylapatite and is evenlydistributed in an oxidic matrix, which gives the material the requiredstrength. The bioactive phase appears as distinct points, the size ofwhich can be allowed to vary depending on the strength of the matrix,but the mean distance between adjacent points must be smaller than 5 μm,preferably smaller than 2 μm.

In the description above for the invention it has been mentioned thatit, within the scope of the inventive concept, also is possible toproduce compacts (bodies), in which different parts have differentcompositions. This will now be illustrated by means of a few possibleexamples. According to a first embodiment of this aspect of theinvention 15 percent by volume hydroxylapatite powder and 85 percent byvolume zirconium oxide powder are mixed. The powder is prepared in thesame manner as is explained in the description above of experimentscarried out. The powder mixture is poured into a polymer can to fill thecan up to half its height. Pure aluminum oxide without any admixture ofany substance is then added to fill the can completely. The can isclosed and the powder is isostaticly compressed in a cold condition at300 MPa. The compact specimen is then isostaticly compressed in a hotcondition at 1225° for 1 h and at a pressure of 160 MPa. The compactspecimen is cut into test bars, in which the center line roughlycorresponds to the boundary between pure aluminum oxide and the aluminumoxide/hydroxylapatite-mixture. The test bars (7 of them) with thedimensions 35×3×3 mm are examined using a three-point-bend-testingmethod. A mean value of the tensile strength was measured to 720 MPa(the lowest value was 540 MPa and the highest 810 MPa).

It is also possible to produce a material similar to the previous one byproducing two separate raw compacts, one of them consisting of a powdermixture of a calcium phosphate powder and an oxidic powder and the otherone solely consisting of an oxidic powder, and combining the rawcompacts through a common isostatic compacting in a hot condition. FIG.3 shows schematicly an example of a work piece produced in this way, inwhich the joint-ball can consist of a ceramic substance made of pureoxide and the stem can consist of a duo-ceramic substance according tothe invention.

FIG. 4 shows schematicly another example. In this instance a pure oxidicpowder 1 is coated with a duo-ceramic powder 2, powder 2 being partiallycovering powder 1, subsequent to which the combined powder isisostaticly compressed in a cold condition and after that is encased andisostaticly compressed in a hot condition.

FIG. 5 shows the opposite instance, namely that a raw compact 3 of aduo-ceramic powder according to the invention partially is coated withan oxidic powder 4, before the compact is consolidated through acombined isostatic pressing in a cold condition followed by an isostaticpressing in a hot condition.

A compact of a duo-ceramic material according to the invention can alsobe treated in order to remove the bioactive phase in the surface area ofa section of the component. This can be done through a chemicaldissolving of the calcium phosphate phase in the surface layer orthrough blasting. In either case small cavities are obtained in thoseareas where the phosphate material previously was present and thesecavities can function as liquid reservoirs, in case implants for jointsare to be produced. Thus, a certain amount of liquid can be kept and inthis way the friction can be reduced within those areas, where a slidingis to take place in the joint. Of course, the remaining parts of theduo-ceramic work-piece are to be left intact in order to be able toutilize the bioactive properties of the duo-ceramic work-piece, wherethis property is desirable.

We claim:
 1. A method for producing a ceramic material having highstrength and bioactive properties and useful as dental and orthopedicimplants, comprising:(1) forming a mixture of matrix powder containing abioinert material selected from the group consisting of titaniumdioxide, zirconium oxide, aluminum oxide and mixtures thereof and abioactive powder containing a bioactive material selected from the groupconsisting of hydroyxylapatite, tricalcuim phosphate and mixturesthereof, wherein 5 to 35% by volume of the bioactive material iscontained in the mixture; (2) forming a raw compact of the mixture; (3)subjecting the raw compact to hot isostatic pressing at pressuresgreater than 50 MPa and at temperatures between 900° C. and 1300° C.sufficiently to density the raw compact to at least 97% of theoreticalmaximum density and to substantially uniformly disperse the bioactivematerial in the bioinert material such that the bioactive material is inparticle form with particle sizes thereof of up to 30 μm and the maximummean distance between particles is about 5 μm.
 2. Method according toclaim 1 wherein the raw compact is densified at a pressure of at least150 MPa and not more than 250 MPa.
 3. Method according to claim 2,wherein the bioinert material is titanium dioxide and the raw compact isdensified at a temperature of 900-1000 ° C.
 4. Method according to claim2, wherein the bioinert material is aluminum oxide and/or xirconiumoxide and the raw compact is densified at a temperature of 1100°-1250°C.
 5. Method according to claim 1, wherein the bioactive material ishydroxylapatite Ca₅ (PO₄)₃ OH.
 6. Method according to claim 1, whereinthe mixture has 10-25 percent by volume of said bioactive materialtherein.
 7. A ceramic material having high strength and bioactiveproperties and useful as dental and orthopedic implants, comprising ahigh temperature, isostatically formed shape of a matrix materialcontaining a bioinert material selected from the group consisting oftitanium dioxide, zirconium oxide, aluminum oxide and mixtures thereofand dispersed material containing a bioactive material selecteg from thegroup consisting of hydroxylapatite, tricalcium phosphate and mixturesthereof, wherein the bioactive material is substantially uniformlydispersed in the matrix material in amounts between about 5 and 35% byvolume and in particle form having particle sizes of up to 30μm withmaximum mean distances between particles of about 5 μm and wherein thedensity of the high temperature isostatically formed shape is at least97% of theoretical maximum density, and wherein the formed shape is hotisostatically pressed at temperatures between 900 °C. and 1300° C. andat a pressure higher than 50 MPa.
 8. Material according to claim 7,wherein the bioinert material is zirconium oxide.
 9. Material accordingto claim 7, wherein the bioinert material is aluminum oxide and themaximum particle size of the bioactive material is 15 μm.
 10. Materialaccording to claim 7, wherein the bioinert material is titanium dioxideand the maximum particle size of the bioactive material is 10 μm.
 11. Abody partially made of a material according to claim 7, wherein one orseveral parts of said body are made of the bioinert material.
 12. Thematerial of claim 7 wherein the maximum distance between particles isabout 2 μm.